The non-Jews used to worship idols and part of their service was pouring wine libations to the deities. Wine that was used in idol worship would be forbidden to drink according to Torah law, and wine merely touched by a non-Jew is forbidden by Rabbinic decree out of concern that they may have poured the wine to an idol. While the Gemorra already acknowledges that non-Jews were no longer dedicated to their idols to present a realistic concern of actually performing the libation, the enactment remains in place.